Why wear a retainer?

I'm using a DTR and am working up to a few hours a day (I take it off when it starts to feel uncomfortable). I've seen people use o-rings as retainers; is this necessary? My understanding of skin growth is that it's permanent; the proof of this is how people who lose large amounts of weight end up with excess skin that can only be removed through surgery. I can commit to a few years of restoring, but not wearing an o-ring the rest of my life. Is this just to keep the new skin over the glans, since it won't stay on its own?